Ultrasonic flow measuring devices are often utilized in process- and automation-technology for detecting volume- and/or mass-flow of a medium through a pipeline. The medium can be a gaseous, vaporous or liquid medium.
Classified on the basis of mounting options, there are two types of ultrasonic, flow measuring devices: Inline, ultrasonic, flow measuring devices, which are usually mounted via flanges into the pipeline, and clamp-on, flow measuring devices, which are applied externally on the pipeline and measure volume- or mass-flow through the wall of the tube, or pipe—thus measuring non-invasively. Clamp-on flow measuring devices are described, for example, in EP 0 686 255 B1, U.S. Pat. No. 4,484,478, DE 43 35 369 C1, DE 298 03 911 U1, DE 4336370 C1 or U.S. Pat. No. 4,598,593.
As regards measuring methods, two different principles are used: Flow measuring devices ascertaining flow via the travel-time difference of ultrasonic, measurement signals in, and opposite to, the flow direction, and flow measuring devices, which win the flow-information from the Doppler shift of the ultrasonic measuring signals. In the case of ultrasonic-measuring devices working on the basis of the travel-time difference method, the ultrasonic-measuring signals are radiated at a predetermined angle, via a coupling element, into, and out of, the pipeline, in which the medium is flowing. In such case, the ultrasonic sensors are usually so arranged, that the traversing sound paths are directed through the central region of the pipeline, or measuring tube, as the case may be. The ascertained, measured value of flow reflects, thus, the average flow of the medium in the pipeline.
In the case of many applications, especially in the case of flow measurements in pipelines of large nominal diameter, this simple averaging is, however, too inaccurate. Alternatively, therefore, it is known to provide a number of pairs of ultrasonic sensors distributed on the periphery of the measuring tube or pipeline, whereby information concerning flow becomes available from different, sectional, angular ranges of the measuring tube pipeline.
The essential component of an ultrasonic sensor is its piezoelectric element. The essential component of a piezoelectric element is a piezoceramic layer metallized in at least one section thereof. Especially, the piezoceramic layer is a film, or a membrane, or diaphragm. By applying an electrical, exciting signal, the piezoceramic layer is caused to oscillate and radiates, via a coupling element, an ultrasonic measurement signal of defined signal form at an angle of incidence into the pipeline. The receiving of the ultrasonic measuring signal after passing through the pipeline is accomplished in reverse manner.
Of course, manufacturing costs for a multi-channel, flow measuring device lie significantly above the costs for a single-channel, flow measuring device.